Exemplary embodiments pertain to aircraft environmental control systems. More particularly, the present disclosure relates to a bypass control of a ram air fan for an aircraft environmental control system.
Environmental control systems (ECS's) supply airflow to an interior of an aircraft to condition, for example, a passenger cabin, crew compartment and/or cargo compartment. In some ECS architectures a ram air fan is utilized to draw ram air into the ECS and across a ram air heat exchanger to condition a fluid flowing through the ram air heat exchanger. The ram air fan is driven by, for example an electric motor or a turbine. The typical air bearing ram air fan is not required to operate during flight for performance reasons, but will uncontrollably windmill during flight, if not driven by an electric motor or turbine. To control ram air fan rotation during flight and to keep the rotating assembly supported by an air film, the ram air fan is typically powered to operate at a specified minimum speed.